1. Field of the Invention
The present invention relates to threshold cancellation means for use in digital mobile radio networks of the type wherein a number of base stations communicate with a number of mobile radio units each being temporarily affiliated to a particular base station.
The present invention relates more specifically to interference cancellation in a direct sequence spread spectrum (DSSS) code division multiple access (CDMA) scheme. Any unwanted signals are demodulated and the recovered data along with the channel information is derived from a pilot signal and is used to create a near replica of the unwanted signal for cancellation.
2. Description of the Related Art
Cancellation can be very effective if the code phase, carrier phase and amplitude of the significant multipath components of the interfering signal are known, provided correct decisions are taken on the data modulating the unwanted signal. Whenever an incorrect decision is taken, tar from canceling the interference the circuit will increase it. For example, if the interference RMS voltage is A volts, then the optimum cancellation would be an equal and opposite, reconstructed signal, also of A volts. However, when an error is made, the output of the cancellation means will not be zero but 2A volts, i.e. providing four times the power which would have been received if cancellation had not been applied. In this situation it would have been better not to have attempted to cancel the interference.